A vehicle operator typically expects a predictable correlation between a turning command and vehicle response. A conventional mechanical steering system, on a vehicle equipped with a single powertrain supplying a common motive power to the drive wheels, typically provides a mechanical link between a steering wheel and the turning wheels of the vehicle. The turning wheels turn in direct response to operator input to the steering wheel, through the mechanical linkage, regardless of whether they are motive wheels (e.g. front wheel drive) or non-motive wheels (e.g. rear wheel drive). Additionally, the motive power to each wheel is typically managed during a turning event to allow the vehicle to achieve a commanded turning radius, using mechanical or hydraulic means.
A vehicle equipped with individually actuated wheel motors provides challenges for systems during steering events, such as negotiating a curve. A significant aspect of successfully negotiating a curve comprises controlling relative wheel speed velocities between inner and outer wheels of the vehicle. Such vehicles include, by way of example, a hybrid vehicle. Regenerative braking systems, antilock braking, and traction control further confound the control of wheel speed during steering events.
Systems employing differential wheel motor speed control in response to a steering input are known in the art. However wheel motor speed control alone does not completely account for variations in vehicle operating conditions or vehicle operation, such as during regenerative braking operation or vehicle acceleration which may occur concurrently with a turning event. Vehicle turning is affected by vehicle operating conditions in that cornering stiffness of a given tire typically varies with operational parameters. Typical operating parameters that are known to affect vehicle turning include tire inflation pressure, vehicle normal load, operator demand for braking effort, lateral force, and suspension characteristics, all of which tend to further vary with operating time, vehicle usage, and ongoing vehicle wear. Furthermore, in a steer-by-wire system, wherein there is no direct mechanical linkage between the steering wheel and the vehicle wheels, any system change in terms of ability to sense operator input or position of the turning wheels, or in responsiveness of the wheel turning motors, may affect vehicle responsiveness to steering input, resulting in either understeer or oversteer. In summary, control of wheel motor speed alone may not provide consistent or sufficient responsiveness in the steering system.
Therefore, what is needed is a system that is able to determine a desired turning radius for a vehicle, and is operable to provide coordinated control of torque and speed of the wheel motors.